multimeter

Recently I found that I was at a loss for my digital multimeter (confession: it’s actually my dad’s, which I’ve been borrowing for an indefinite period), and I needed to test the continuity on a PCB. So I quickly scrapped together some parts to make a simple light-up continuity meter.

Continuity meters are used to test whether two components or joints in a circuit are continuous, or directly connected. Most multimeters have a continuity meter built-in. When you touch the meter’s test probes to 2 different, yet connected spots on a breadboard or PCB, the meter will either beep or light up.

My continuity meter lights up when the test probes are connected. I utilized a small breadboard, three alligator clips, a super bright white LED, two needles, electrical tape, and a 330Ω resistor to make my meter. I didn’t have to use any solder, and after I used the excess wire to wrap the breadboard and the battery together, it came together nicely. It works well!

Here’s how to make your own!

Materials Needed

You’ll need the following, depending on how you want to make your continuity meter and what parts you have.

alligator clips and breadboard (or soldering iron)

x2 needles (act as test probes, if you want something more precise than the wire as a test probe)

x1 battery or combination of batteries with a combined voltage of 1.5 to 9V, the total voltage being greater than but as close as possible to the voltage drop of the LED

x1 resistor, ohm value discussed later

x1 LED, you’ll want to know its voltage drop (see here if you don’t know, you’ll have to scroll down a bit)

electrical tape, duct tape, or glue for keeping the meter together after assembly

Assembly

The hardest part of building your continuity meter is figuring out the right resistor value to use with the LED. For that, will use a bit of math and Ohm’s Law. You’ll need to know the voltage of your batteries (Vcc) and the voltage drop of your LED (Vd). The forward current limit (I) on most LEDs can be approximated to 0.02A or 20mA, so we’ll use that as our current value. This is how to calculate the resistance (R) of the resistor you’ll need:

V = IR

R = V/I

R = (Vcc – Vd) / I

R = (Vcc – Vd) / 0.02

For my continuity meter, I had an old 9V battery that measured at 8V and a white LED with a forward voltage drop of about 3V. So I needed an ((8V – 3V) / 0.02mA) = 250 Ω resistor. The closest standard value resistor up is 330Ω, which is what I used.

Next comes assembly. Whether you’re using a soldering iron or a breadboard, it’s simple. Either solder your ‘test probes’ to the wire, or put the needles directly into the alligator clips. Take a look at the diagram below for guidance.

That’s about it. The LED should light up when you touch the two test probes! To containerize your continuity meter, duct tape the battery and excess wires to the breadbread, or if you soldered it, you can try gluing it together.

Important Disclaimer about using this continuity meter:

Be careful using your continuity meter. I’d recommend using it only for testing boards and components where you know that things are supposed to be continuously connected. You don’t want to use it as you’re trying to figure out what a board does, you run the risk of shorting out components on the board. Definitely don’t use your homemade continuity meter on circuits that have live power!